Radio frequency identification (RFID) transponders (tags) are usually used in conjunction with an RFID base station, typically in applications such as inventory control, security, access cards, and personal identification. The base station transmits a carrier signal that powers circuitry in the RFID tag when the RFID tag is brought within a read range of the base station. Data communication between the tag and the station is achieved by modulating the amplitude of the carrier signal with a binary data pattern, usually amplitude shift keying. To that end, RFID tags are typically integrated circuits that include, among other components, antenna elements for coupling the radiated field, rectifiers to convert the AC carrier signal to dc power, and demodulators to extract the data pattern from the envelope of the carrier signal.
If fabricated at sufficiently low cost, RFID tags can also be useful in cost-sensitive applications such as product pricing, baggage tracking, parcel tracking, asset identification, authentication of paper money, and animal identification, to mention just a few application. RFID tags could provide significant advantages over systems conventionally used for such applications, such as bar code identification systems. For example, a basket full of items marked with RFID tags could be read rapidly without having to handle each item, whereas they would have to be handled individually when using a bar code system. Unlike bar codes, RFID tags provide the ability to update information on the tag. However, the RFID technology of today is too expensive for dominant use in such applications. There are several factors that drive up the cost of RFID tags, the most significant of which is the size of the silicon integrated circuit that makes up the tag.
FIG. 1 shows a conventional rectifier utilizing a diode bridge 2. Antenna element 4 requires two connections 6, 8 on opposite sides of diode bridge 2. The rectified signal is output at node 10.
FIG. 2 shows another conventional rectifier utilizing a MOSFET bridge 12. Antenna element 4 also requires two connections 6, 8 on opposite sides of MOSFET bridge 12. In order to accommodate these connections 6, 8, conventional RFID tags require at least two pads large enough to bond wire for the attachment of an external antenna coil 4. Since RFID tag chips are generally relatively small, these pads consume a significant percentage of the integrated circuit area of a conventional RFID tag.
Another concern with conventional RFID tags is the maximum operating reading distance from the base station. In both examples of prior art shown in FIGS. 1 and 2, the rectified output signals at node 10 are only one half the peak-to-peak voltage of the corresponding resonating nodes 6. The amplitudes of the signals on output nodes 10 are related to the maximum operational distance between the RFID tag and the base station.